docs: def write(self, feature, value, *args, **kwargs):
          """Same as  :py:meth:`~chemlab.io.iohandler.IOHandler.read`. You have to pass
          also a *value* to write and you may pass any additional 
          arguments.
          
          **Example**
          
          ::
          
              class XyzIO(IOHandler):
                  can_write = ['molecule']
          
                  def write(self, feature, value, *args, **kwargs):
                      self.check_feature(feature, "write")
                      if feature == 'molecule':
                         # Do stuff
                         return geom
          
          """
          if 'w' not in self.fd.mode and 'x' not in self.fd.mode:
              raise Exception("The file is not opened in writing mode. If you're using datafile, add the 'w' option.\ndatafile(filename, 'w')")
          
          self.check_feature(feature, "write")